1. Field of the Invention
The present invention provides radiation shields for cylindrical superconducting magnets comprising a plurality of axially aligned coils of superconducting wire. In particular, the present invention relates to superconducting magnets which are not cooled by immersion in a liquid cryogen, but are arranged for cooling by thermal conduction through a cooled surface in mechanical contact with the coils.
2. Description of the Prior Art
Cylindrical superconducting magnets are commonly employed in magnetic resonance imaging (MRI) systems. A strong, homogeneous background magnetic field is provided by the superconducting magnets, and a gradient coil assembly is typically provided inside the bore of the superconducting magnet, and generates rapidly-varying magnetic field gradients in three orthogonal dimensions.
The superconducting magnet is cooled to cryogenic temperatures, and it is conventional for the magnet to be housed inside a hollow cylindrical outer vacuum chamber (OVC), and for one or more hollow cylindrical thermal radiation shields to be positioned between the superconducting magnet and the OVC. Typically, the thermal radiation shield, or the outer thermal radiation shield if there is more than one, is cooled to a temperature of approximately 50K. The assembly therefore comprises at least two closely-stacked cylindrical tubes within the bore of the magnet.
The time-varying currents applied to the gradient coils during an MRI imaging sequence interact with the homogeneous background magnetic field to cause Lorentz forces to act on the gradient coils, resulting in vibration of the gradient coil assembly.
The time-varying magnetic fields generated by the gradient coils induce eddy currents in the material of nearby conductive surfaces, such as bore tubes of the OVC and the thermal radiation shield(s). These eddy currents, flowing through the resistive material of the bore tubes, cause heating which may risk magnet quench due to a rise in coil temperature. Little heat energy is required to cause a quench, due to the very low heat capacity of solid materials at 4K, a typical temperature of operation of cylindrical superconducting magnets.
Furthermore, the eddy currents induced in the conductive bore tubes will interact with the homogeneous background magnetic field to cause Lorenz forces to act on the bore tubes, resulting in vibration of the bore tubes. These vibrations interact with the homogeneous background magnetic field and cause further (secondary) eddy current generation within the bore tubes, in turn causing heating and inducing further (tertiary) eddy currents in neighboring conductive surfaces through generation of secondary magnetic fields. The tertiary eddy currents will in turn produce tertiary magnetic fields. In addition to the unwelcome heating already described, the vibrations cause unpleasant noise for patients placed within the bore of the magnet for imaging.
The eddy currents produced in the material of the OVC bore tube will help to shield the bore tube of the thermal radiation shield stray magnetic fields from the gradient coils.
In a liquid cryogen bath cooled magnet, the liquid cryogen such as helium provides adequate cooling of the coils to prevent a quench happening as a result of eddy currents. However, more recent designs of superconducting magnets do not have coils cooled by immersion in liquid cryogen. Rather, the coils are arranged for cooling by thermal conduction through a cooled surface in mechanical contact with the coils. The cooling may be provided with a cryogen-filled thermosiphon, or by a cryogenic refrigerator thermally linked to the cooled surface. In such low-cryogen inventory magnets, the problem of unwanted gradient coil induced heating (GCIH) is urgent because the thermal resistance between the coil and the cooled surface is significantly higher than in the former liquid cryogen cooled magnets.
The following patent publications relate generally to gradient coil interaction and the reduction of noise and vibration: DE 10 2006 000 923 B4; U.S. Pat. No. 7,535,225 B2; JP 2005279187 A; and US 2006/0266053 A1.